Silicon carbide has excellent voltage withstanding characteristics in comparison with silicon single body. Therefore, it has been proposed that large power consumption semiconductor devices such as a power transistor be implemented by a silicon carbide semiconductor device using a semiconductor substrate formed of silicon carbide.
Furthermore, it has been proposed for such a silicon carbide semiconductor device that the electrode formed on a surface of a semiconductor substrate of silicon carbide be formed of nickel (Ni). The nickel can form an ohmic contact with the surface of the semiconductor substrate formed of silicon carbide while reducing the resistance of the contact portion.
However, when oxide film is formed on the semiconductor substrate of silicon carbide and a contact hole is formed in the oxide film and the contact hole is filled with nickel, nickel thin film is also formed on the upper surface of the oxide film. The nickel thin film formed on the oxide film frequently becomes peeled off, which lowers the reliability of the silicon carbide semiconductor device.
Therefore, it has been proposed in JP-A-10-125620 (hereafter patent document 1) that the nickel thin film on the oxide film be etched by a lithography technique. However, because the mask may be formed in an erroneous location, it is difficult to properly leave the nickel thin film at only the contact hole portion. More fully, if over-etching is performed subsequent to the lithography step, the nickel thin film in the contact hole is etched, so that the contact area is reduced and the contact resistance is increased. Further, when under-etching is carried out, nickel remains on the oxide film.
Therefore, it has been proposed in JP-A-2000-12846 (hereafter Patent Document 2) that the silicon carbide and nickel be chemically combined with each other in self-alignment at the contact portion with the surface of the semiconductor substrate by a heat treatment after the nickel thin film is formed. Using this heat treatment eliminates the problems associated with the lithography technique disclosed in Patent Document 1 and discussed above. However, as indicated in Patent Document 2, the heat treatment results in the problem of the nickel thin film becoming particles in a subsequent step. Further, it has been confirmed by the inventors that when the nickel thin film at the contact hole is chemically combined with the semiconductor substrate of silicon carbide by the heat treatment and the nickel thin film which is not chemically combined with silicon carbide is then removed by ultrasonic cleaning, the nickel thin film remains on the oxide film.
The contents of JP-A-10-125620 and JP-A-2000-12846 are incorporated herein by reference.